It is known that internal combustion engines, and in particular Diesel engines, are equipped with exhaust gas aftertreatment systems. Aftertreatment systems treat exhaust gases that exit the combustion chamber and that are directed into an exhaust pipe having one or more aftertreatment devices configured to filter and/or change the composition of the exhaust gases, such as for example an Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), a Lean NOx Trap (LNT), and/or a Selective Catalytic Reduction (SCR) system or a SDPF (SCR on Diesel Particulate Filter).
The SCR is a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gas are reduced into diatomic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3), that is absorbed inside the catalyst. The ammonia is obtained through thermo-hydrolysis of a Diesel Exhaust Fluid (DEF), typically urea (CH4N2O) that is injected into the exhaust gas pipe through a dedicated injector located between the DPF and the SCR.
For future diesel passenger cars, Euro and US emission legislations will require a greater reduction of exhaust emission in terms of NOx, compatibly with an increase of fuel economy (CO2 reduction). This goal requires a redesign of the current aftertreatment architecture. in particular, the integration of SCR. functionalities within a filter substrate (SDPF) provides vehicles that are cleaner, more efficient and more capable of obtaining fuel savings. This is the reason why recently, Selective Catalytic Reduction wash coated particulate filters (also referred to as SDPFs) have been introduced in the aftertreatment system architecture.
A SDPF is an SCR (Selective Catalytic Reduction) catalyst coated on a porous DPF (Diesel Particular Filter). However, these technical developments lead to more complex aftertreatment systems which, in combination with the current stringent regulation requirements, require a dedicated management, in particular during filter regeneration processes when the exhaust gas temperature are increased above 600° C. to efficiently burn the loaded soot/particulate matter stored inside the particulate filter. More specifically the presence of the SCR coating into the filter substrate requires a very precise control of the temperatures inside the filter, in order to avoid any potential damage or over-aging of the SCR coating, damage that may occur typically if temperatures reach values above 850° C.-900°°C.
It may be necessary to remove the particulate matter or soot that progressively accumulates inside the filter to prevent the pressure drop across the filter from becoming excessive in order to guarantee and/or restore the efficiency of the particulate filter. This process, which is conventionally known as DPF regeneration, is achieved by increasing the temperature of the exhaust gases entering the DPF (typically up to 630° C.), which in their turn heat the filter up to a temperature at which the accumulated particulate burns off.
A known strategy to increase the exhaust gas temperature provides for the exhaust gases to be mixed with a certain amount of unburned fuel (HC) that oxidizes in the oxidation catalyst, thereby heating the exhaust gases that subsequently pass through the DPF. The unburned fuel may come from the engine cylinder thanks to the so called after injections or may be supplied by means of a dedicated fuel injector, which may be located directly in the exhaust pipe, for example upstream of the DOC. During regeneration processes specific conditions might occur which could led to very high temperature events as consequence of soot burning. These conditions are represented by very high amount of soot loaded (for example for values of soot greater than 8 g/l) and by instantaneous reduction of exhaust flow rates with still enough oxygen to burn soot.
Currently, no defined strategy to predict an unexpected over-temperature event, also known as Drop To Idle (DTI) event, during the filter regeneration inside the SDPF component is known.